1. Field of the Invention
The present invention relates to an ink, and a recording process making use of it. More particularly, it relates to a water-based ink capable of giving a black image improved in indoor color change resistance, and a recording process, in particular, ink-jet recording process, making use of it.
2. Related Background Art
Water-based inks comprising a water-soluble dye dissolved in a water-based medium have been hitherto used as inks used in fountain pens and felt pens and inks used for ink-jet recording. In these water-based inks, water-soluble organic solvents are commonly added so that pen points or ink ejection nozzles can be prevented from being clogged with ink.
It is required for these conventional inks to give an image with a sufficient density, not to cause any clogging at pen points or nozzles, to have good drying properties on recording mediums, to cause less feathering, to have excellent shelf stability, and, particularly in ink-jet recording systems utilizing heat energy, to have excellent thermal resistance. It is also required for the image formed to have a satisfactory light-fastness and water fastness.
Inks with various hues are also prepared from dyes with various hues. Of these, black inks, which are used in both monochromatic and full-color images, are most important inks. As dyes for these black inks, C.I. Food Black 2 has been mainly used taking account of various performances (see Japanese Laid-Open Patent Application No. 59-93766 and No. 59-93768).
Among the various required performances, what is particularly important is the fastness of the images formed.
In regard to the fastness of images, hitherto mainly questioned is the color fading due to direct sunlight or every kind of illumination light. Such a problem of color-fading has been attempted to be settled by the selection of dyes having superior light-fastness.
Recently, however, a problem of color changes of images has become important in addition to the above color fading. Namely, images formed by conventional inks have not only the problem of color fading but also the problem of color changes. The color changes refer to changes in hues with, however, less changes in density, and what is important in black inks particularly used in a largest quantity is a problem of the browning that black turns brown. In particular, in the instance of full-color images, this browning results in a great lowering of image quality.
This problem of browning also occurs indoors without exposure to direct sunlight. The color change is also accelerated depending on the types of recording mediums on which images are formed, and this problem of browning has been unavoidable in respect of the C.I. Food black 2 that has been hitherto widely used.
In particular, so-called coated papers comprises a substrate such as paper and formed thereon an ink-receiving layer containing a pigment and a binder, for the purpose of improving the color-forming performance of ink and the image quality such as sharpness and resolution. In such an instance, the browning may seriously occur even with use of inks that may cause less problem of color change in the instance of plain papers. This problem has been unsettled by the mere selection of dyes having superior light-fastness.